Thirty seconds initially spent framing paintings of birds (7)

'birds' is the definition.
(martin is a kind of bird)

'thirty seconds initially spent framing paintings' is the wordplay.
'thirty seconds' becomes 'min' (I've seen this in other clues).
'initially' indicates taking the first letters.
'framing' means one lot of letters goes inside another (inserted letters are framed by the outside letters).
'paintings' becomes 'art' (painting is a type of artwork).
The initial letter of 'spent' is 's'.
'min'+'s'='mins'
'mins' placed around 'art' is 'MARTINS'.

'of' acts as a link.